1. Field of the Invention
This invention relates to a multi-piece solid golf ball comprising a solid core and a cover of two inner and outer layers enclosing the solid core and more particularly, to a multi-piece solid golf ball best suited for those players with a head speed of about 50 m/s.
2. Prior Art
At present, golf balls of various structures have been proposed. A greater number of proposals are made on solid golf balls, especially multi-piece solid golf balls having a solid core enclosed with a cover of plural layers, so as to improve their travel distance, controllability or spin rate and feel when hit. See JP-A 4-244174, JP-A 6-142228, JP-A 7-24084, JP-A 7-24085, JP-A 9-10358, and JP-A 11-104273.
However, there is a desire to have a multi-piece solid golf ball having further improved flight performance, good spin properties, and a pleasant feel when hit with wood, iron and putter clubs as well as good scraping resistance and durability.
Making extensive investigations to meet the above demand, the inventor has found that a multi-piece solid golf ball comprising a solid core and a cover of two inner and outer layers enclosing the solid core and formed with a plurality of dimples is improved when the solid core has a deflection of at least 1.1 mm under an applied load of 294 N (30 kgf) and a surface and a center, the surface-center hardness difference given as the JIS-C hardness on the core surface minus the JIS-C hardness at the core center being greater than 15 units, the cover inner layer is formed of a resin composition comprising as essential components, specific amounts of (a) an olefin-unsaturated carboxylic acid random copolymer, olefin-unsaturated carboxylic acid-unsaturated carboxylic ester random copolymer, or a metal ion neutralized product of the copolymer, or a mixture of the copolymer and the neutralized product, (b) a fatty acid having a molecular weight of at least 280 or derivative thereof, and (c) a basic inorganic metal compound capable of neutralizing acid groups in components (a) and (b), the cover inner layer has a Shore D hardness of 45 to 65, the cover outer layer has a Shore D hardness of 35 to 55, the cover outer layer is softer than the cover inner layer, and the sum of dimple trajectory volumes each obtained by multiplying the volume of a dimple by the square root of a dimple diameter is 530 to 750.
Specifically, the invention provides a multi-piece solid golf ball as defined below.
(1) A multi-piece solid golf ball comprising a solid core and a cover of two inner and outer layers enclosing the solid core, wherein
the solid core has a deflection of at least 1.1 mm under an applied load of 294 N (30 kgf) and a surface and a center, the surface-center hardness difference given as a JIS-C hardness at the core surface minus a JIS-C hardness at the core center being greater than 15 units,
said cover inner layer is formed of a resin composition comprising as essential components,
(a) 100 parts by weight of an olefin-unsaturated carboxylic acid random copolymer, an olefin-unsaturated carboxylic acid-unsaturated carboxylate random copolymer, a metal ion neutralized product of each said copolymer, or a mixture of each said copolymer and the neutralized product,
(b) 5 to 80 parts by weight of a fatty acid having a molecular weight of at least 280 or derivative thereof, and
(c) 0.1 to 10 parts by weight of a basic inorganic metal compound capable of neutralizing acid groups in components (a) and (b),
said cover inner layer has a Shore D hardness of 45 to 65, said cover outer layer has a Shore D hardness of 35 to 55, said cover outer layer is softer than said cover inner layer, and
the sum of dimple trajectory volumes each obtained by multiplying the volume of a dimple by the square root of a dimple diameter is 530 to 750.
(2) The multi-piece solid golf ball of claim 1 wherein said resin composition has a melt index of at least 0.5 dg/sec.
(3) The multi-piece solid golf ball of claim 1 or 2 wherein said cover outer layer is mainly formed of a thermoplastic or thermosetting polyurethane elastomer, polyester elastomer, ionomer resin, polyolefin elastomer or a mixture thereof.
(4) The multi-piece solid golf ball of claim 3 wherein said cover outer layer is mainly formed of a thermoplastic polyurethane elastomer obtained using an aromatic or aliphatic diisocyanate.
(5) The multi-piece solid golf ball of claim 3 wherein said cover outer layer is mainly formed of the reaction product of a thermoplastic polyurethane elastomer with an isocyanate compound.
(6) The multi-piece solid golf ball of any one of claims 1 to 5 wherein component (b) in said resin composition is a fatty acid containing 18 to 80 carbon atoms in the molecule or derivative thereof.
(7) The multi-piece solid golf ball of any one of claims 1 to 6 wherein at least 50 mol % of the acid groups in the essential components (a) and (b) are neutralized with metal ions.
(8) The multi-piece solid golf ball of any one of claims 1 to 7 wherein said cover inner layer is formed of the resin composition comprising up to 100 parts by weight of an olefinic elastomer or polyester elastomer per 100 parts by weight of the essential components (a) to (c) combined.
(9) The multi-piece solid golf ball of any one of claims 1 to 8 further comprising an adhesive layer between said cover inner and outer layers.
(10) The multi-piece solid golf ball of claim 9 wherein the adhesive layer is mainly formed of a urethane resin base adhesive or chlorinated polyolefin base adhesive to a thickness of 0.1 to 30 xcexcm.
(11) The multi-piece solid golf ball of any one of claims 1 to 10 wherein said solid core has a specific gravity of 1.0 to 1.3, said cover inner layer has a specific gravity of 0.8 to 1.5, and said cover outer layer has a specific gravity of 0.9 to 1.3.
(12) The multi-piece solid golf ball of any one of claims 1 to 11 wherein provided that a dimple has a diameter and a depth in elevational cross section at its center, S1 is the cross-sectional area of the dimple and S2 is an area given by multiplying the diameter by the depth, an average SA of dimple cross-sectional area ratios given by S1/S2 is from 0.58 to 0.68, and the total number of dimples is 360 to 540.
The golf ball of the invention has a stretching flight performance along a relatively low trajectory, an increased travel distance, ease of control on iron shots, and a pleasant feel when hit with wood, iron and putter clubs, and is resistant to scraping and durable upon control shots with an iron club.
The multi-piece solid golf ball of the invention gives a soft feel and good rebound owing to the increased hardness distribution of the core and is best suited for play by those players with a head speed of about 50 m/s. As the same time, the cover inner layer is formed of the resin composition which is improved in thermal stability, flow and moldability and can impart satisfactorily improved resilience, and adjusted to an adequate hardness, so that the cover inner layer may cooperate with the cover outer layer and the solid core to achieve drastic improvements in rebound, durability and controllability.